1. Field of the Invention
This invention relates to an engine overheat detection system and more particularly to an improved engine overheat detection system that is most suitable to a marine engine.
2. Description of Related Art
Watercraft powered by inboard or outboard motors typically include an electrical system. The motor includes a water propulsion device which is powered by an internal combustion engine. As is well known, an ignition system is utilized to fire one or more ignition elements corresponding to each combustion chamber of the engine, igniting the air and fuel mixture in each combustion chamber of the engine.
These engines commonly include a liquid cooling system. Liquid coolant in the form of water in which the watercraft is operating is supplied to various cooling passages or jackets associated with the engine. In some instances, the cooling system is arranged such that the coolant drains from the coolant jackets when the engine is stopped.
In order to prevent engine overheating, an overheat detection system may be associated with the engine. The detection system includes a sensor for sensing the temperature of the engine. The output of the sensor may be used by an engine control unit to shut off the engine by disabling the ignition system.
This system has the drawback that at certain times a condition of engine overheat may be indicated when in fact the engine is not in an overheat condition. This drawback is likely to happen particularly in connection with an engine that operates on a four stroke principle. Because such a four stroke engine has an oil pan therein for lubrication and lubricant contained in this oil pan tends to accumulate much heat during the engine operation.
Referring to FIG. 1, when the engine is operating normally and coolant is in the water jacket(s), the temperature inside the water jacket Tw remains lower than a predetermined high temperature or threshold temperature Tlim (85.degree. C. in FIG. 1). When the engine is shut off, however, the coolant drains from the jacket. In addition, the temperature To of the lubricant contained in the oil pan is still high for some time after the engine is stopped. Because the lubricant temperature To is around 130.degree. C. when the engine is running and the temperature To is hard to fall down. Since no coolant remains in the water jacket and the lubricant temperature To is high, the temperature in the jacket rises immediately after the engine has been stopped. The temperature may rise to a point well above the predetermined high temperature Tlim. Then, with the lubricant temperature To falling down, the temperature inside the water jacket Tw falls back below the temperature Tlim.
If the engine is subsequently restarted before the temperature in the jacket Tw falls back below the temperature Tlim, the overheat detection system will indicate that the engine is overheated. This is due primarily because coolant is not yet being supplied to the cooling jacket(s).
In order to prevent the wrong determination of overheat from being occurring when the engine is restarted immediately after being stopped, one idea may be proposed wherein no overheat detection is made during a predetermined time after the engine is started. FIG. 2 shows a flowchart of an overheat detection routine in accordance with this idea as an example.
Immediately after the engine is started, the program goes to a step S1 and checks if an overheat sensor (thermal switch) is on or off. If it is on, i.e., the temperature inside the water jacket Tw is higher than the predetermined high temperature Tlim, the program goes to a step S2 to determine if the engine has been just started or not. This state is represented by that the engine speed is less than 2000 rpm. If this is negative, the program goes to a step S3 and prevents an overheat signal from being output for 20 seconds. Then, the program goes to a step S4 to check again with the overheat sensor if it is still on. If it is positive, the program permits to output an overheat signal in a step S6. Meanwhile, if the engine speed is equal to or greater than 2000 rpm in the step S2, the program goes to a step S5 and prevents the overheat signal from being output for 90 seconds. Thus, the wrong determination of overheat is prevented. The method and system for this overheat detection will be described more in detail later.
However, another problem arises if the prevention time (indicated as Ts in FIG. 1) is relatively long. That is, in the event an actual overheat happens, no overheat signal is provided during the prevention time and the engine must operates under this overheat condition for a while.
It is, therefore, a principal object to provide an improved engine overheat detection system which overcomes the above-stated problems.